IODP

doi:10.2204/iodp.pr.335.2011

Operations

Drilling strategy for Hole 1256D

Leg 206 and Expedition 309/312 made significant progress in recovering an intact section of the upper oceanic crust through the erupted lavas and sheeted dikes and into the uppermost gabbros. Average rates of recovery and penetration are summarized in Figure F18 and Table T2. The inverse relationship between spreading rate and depth to axial melt lenses has been confirmed, supporting the strategy of drilling in crust formed at a superfast spreading rate to achieve the first upper crustal penetration. However, fundamental questions regarding the formation of ocean crust remain. The principal goal of Expedition 335’s return to Hole 1256D was to deepen the hole sufficiently into plutonic rocks (a few hundred meters) to obtain definitive answers to long-standing questions about the structure and composition of the oceanic crust and about mechanisms of crustal accretion (these ideas are developed in greater detail in “Background and geological setting”).

Operational planning for Expedition 335 was informed by three principal sources:

  1. The Operations teams experiences during Leg 206 and Expedition 309/312 to Hole 1256D and previous deep basement coring by scientific ocean drilling;

  2. Aspects of the IODP Operations Review Task Force Meeting “Expeditions 309/312 Superfast Spreading Rate Crust” (ORTF-3 [www.iodp.org/ortf/], June 2006); and

  3. A US Implementing Organization (USIO) position paper, “Operational requirements for returning to Hole 1256D” (September 2006).

The recommendations of the Expedition 309/312 Review Task Force indicate an understanding of the nonstandard requirements of deep basement drilling and echo the suggestions made by the Co-Chief Scientists of those expeditions. Three recommendations pertinent to operations during Expedition 335 were

  1. To investigate alternate scheduling strategies (e.g., at-sea crew changes, lengthening standard expedition durations to maximize on-site time for deep drilling objectives; ORTF-309/312-03),

  2. To investigate and prioritize avenues for enhancing coring/drilling capability (particularly of hard formations) for deep-drilling programs (ORTF-309/312-11), and

  3. To build on the experiences of Phase 1 expeditions and actively explore future applications of drilling muds (particularly those with heavy lifting capability) for riserless hole cleaning and stabilization (ORTF-309/312-12).

Unfortunately, there has been only modest progress on these recommendations in the 5 years since the ORTF-309/312 meeting, with a half-length expedition initially scheduled by the Science Advisory Structure for the return to Hole 1256D and an absence of new drilling and coring options for hard formations and hole cleaning.

The IODP Science Planning Committee requested in 2006 (SPC Consensus 0603-19) that the USIO identify the operational requirements for further drilling in Hole 1256D. Texas A&M University (TAMU) engineers from the USIO presented their operational plan to an audience of scientists and independent drilling engineers at the Mission Moho Workshop (Portland, Oregon, 7–9 Sept 2006) for technical review. There was consensus support for the plan proposed.

The USIO considered four deepening scenarios:

  1. Resume rotary core barrel (RCB) coring in Hole 1256D using large-volume (100–150 bbl) high-viscosity mud sweeps combined with frequent bit trips,

  2. Enlarge the hole to 18½ inches to isolate the out-of-gauge section using 13⅜ inch casing,

  3. Forego 13⅜ inch casing and enlarge the hole to 14¾ inches to isolate the out-of-gauge section using 10¾ inch casing, and

  4. Offset and start a new hole following the casing strategy employed during Leg 206.

Two key points were noted by the USIO and independent engineers at the Mission Moho Workshop:

  1. Hole 1256D is in excellent condition, and remedial engineering operations such as reaming and casing are premature.

  2. Neither the offshore industry nor scientific ocean drilling operators have ever attempted to open up an existing deep basement hole to any significant depth (hundreds of meters) in basalt and insert casing.

Regardless of the casing strategy proposed (2 or 3), attempting to open any portion of Hole 1256D to accommodate casing would require significant new hardware, numerous pipe trips, and extend over several expeditions. Such operations would require unproven technology and would be extremely challenging with substantial risk of irreparable damage to Hole 1256D without any further coring or recovery. The USIO and independent experts felt that approaches 2 and 3 are not viable options and recommended that they were not considered further.

The preferred USIO strategy was 1—resuming RCB coring with frequent large-volume high-viscosity mud sweeps (ORTF-309/312-12). During Expedition 309/312, large-volume mud sweeps were effective at clearing cuttings from the hole. However, because of the end of Phase I operations, mud stock depletion precluded the vigorous implementation of this approach throughout the whole of Expedition 312. The refitted JOIDES Resolution should be equipped to handle the necessary supplies to maintain an aggressive mud program. Following the recommendations of the IODP Expedition 309/312 Operations Review Task Force, the JOIDES Resolution was stocked with at least 60 T of sepiolite and attapulgite for use during Expedition 335.

Assuming reasonable hole conditions and an average rate of penetration (ROP) comparable to previous coring in this hole, TAMU engineers estimated that this approach should deepen Hole 1256D ~350 m with 39 days on site (Teagle et al., 2010). The engineering and design lessons learned from a few hundred meters deeper penetration of intact ocean crust would be invaluable in planning and exploiting future deep penetration targets.

Further deepening of Hole 1256D using large mud sweeps is the only strategy that begins with coring, is the most likely to return samples from deeper in the hole, and has the largest possibility of building on the tremendous success of previous deepening operations on Expedition 309/312 that employed proven drilling techniques.

Before conditioning the hole for further deepening, an attempt would be made to acquire an equilibrium temperature profile and recover a water sample before the thermal structure of the crust is perturbed by cleaning and drilling operations (see next section). We would reenter the hole with a tricone drilling bit on a bit release and slowly descend past the zone around 900 mbsf that caused an obstruction during Expedition 312. If the hole was clear, we would withdraw the pipe, drop the bit, and reenter to below the rat hole to allow logging with the triple combo, including the Modular Temperature Tool (MTT). If the wireline conditions were suitably benign, we would attempt to take a borehole water sample from near the bottom of the hole using the water-sampling temperature probe (WSTP).

Expedition 335 operations

Port call

All Expedition 335 scientists transferred from their hotel in San Jose, Costa Rica, to Puntarenas, Costa Rica, and moved onto the vessel on Thursday, 14 April 2011, except for one whose travel was delayed by 3 days. The morning of Friday, 15 April, the Co-Chief Scientists engaged in a press conference at the Marine Biology Station of the Universidad National Costa Rica, located near the vessel. Regional visitors toured the ship in the afternoon. The final science party member arrived on board on Saturday, 16 April.

Transit

The last line was released on 17 April 2011 at 0420 h, and the vessel began transit to Site 1256 (644.2′N, 9156.1′W) at 0430 h. The 478 nmi voyage to Hole 1256D was accomplished at an average speed of 10.6 kt. The vessel was positioned on the established coordinates of the hole at 0115 h on 19 April. The water depth established during Leg 206 was 3645.4 meters below rig floor (mbrf). The final depth of the hole at the end of Expedition 312, when it was last visited in December 2005, was 1507.1 mbsf. Below is a narrative of the operations conducted in Hole 1256D from this time (Table T3).

Opening Hole 1256D (19–23 April)

Run 1

Hole 1256D was reentered with a hard formation Smith 9⅞ inch F9 tricone bit (Fig. F19A) affixed to a mechanical bit release (MBR) and four-stand bottom-hole assembly (BHA). The initial objective was to test the openness of the hole to 1100 mbsf without circulation or rotation so that, if clear, an equilibrium temperature profile could be run to 1350 mbsf using the MTT logging suite. This would have been followed by a borehole water/microbiology sampling run using the WSTP according to plan (Teagle et al., 2010).

The drill string was lowered to ~920 mbsf, where it encountered a ledge precluding the planned wireline operations. This interval had impeded smooth transit of the drill string during Expedition 312. The wireline caliper, FMS, and Ultrasonic Borehole Imager (UBI) logs from that expedition indicate an eroded zone from ~920 to 935 mbsf (see “Observations of highly out-of-gauge interval (920–960 mbsf)” for more detail). Starting at 2330 h on 19 April 2011, and for 32 h until 0600 h on 21 April, the drillers attempted to work past the obstruction without success, and the drill string was recovered.

Run 2

A more aggressive Reed 9⅞ inch tricone bit (International Association of Drilling Contractors [IADC] Type 517) (Fig. F19B), usually employed in softer formations, was mounted along with a tandem set of junk basket subs, and Hole 1256D was reentered for the second time at 0105 h on 22 April 2011. Drilling on the bridge at ~922 mbsf resumed at 0445 h and continued to 2100 h (16.3 h) without discernible progress. The drill string was recovered, with the bit clearing the rotary table at 0605 h on 23 April. The contents of the junk baskets were examined and found to contain angular coarse sandy basaltic chips along with a few small (2–3 cm) subrounded basaltic pebbles, consistent with the rocks cored when this interval was first drilled during Expedition 309. After discussing options with the on-board scientific leadership, it was decided that placing a cement plug at and above the bridge could possibly stabilize the zone and allow us to advance past the obstruction.

Cementing and reopening Hole 1256D (23 April–3 May)

Run 3

A cementing assembly was made up of the used Reed 9⅞ inch tricone bit without jets and two stands of drill collars. Hole 1256D was reentered for the third time at 1520 h on 23 April 2011. After the driller tagged the bridge with the bit at 922 mbsf, the circulating head was made up to the drill string and tested to 1500 psi. Five bbl of 16 ppg (pounds per US gallon; specific gravity ~ 1.9) blended cement was pumped into the hole and chased with a column of seawater equal to the volume of the drill string. The drill pipe was then pulled back to 807 mbsf, where an additional seawater flush consisting of three drill string volumes was pumped through the system. After rigging down the cementing equipment, the drill pipe was recovered, with the bit clearing the rotary table at 0515 h on 24 April.

Run 4

The Reed bit was replaced with a new 9⅞ inch tricone bit (Atlas HP61), and the drill string was deployed to 1596 mbrf. Hole 1256D was reentered for the fourth time at 1655 h on 24 April 2011 to test the result of the cementing operation. The bit tagged the bridge at 922.0 mbsf, indicting that the 5 bbl of cement deployed earlier was not sufficient to fill the cavity above the obstruction. The driller’s effort to wash and ream past the ledge met with high erratic torque and was given up after 1 h. The drill string was tripped to the surface, with the bit clearing the rotary table at 0615 h on 25 April.

Run 5

The cementing assembly was deployed for the second time, with the bit entering the reentry cone for the fifth time at 1445 h on 25 April 2011. The bit was placed at 922 mbsf, and this time 50 bbl of 15 ppg cement was pumped into the hole. The bit was recovered at 0345 h on 26 April.

Run 6

A drilling assembly with an Atlas tricone bit was redeployed, and at 1520 h on 26 April 2011, Hole 1256D was reentered for the sixth time during this expedition. The top of the cement plug was tagged at 882 mbsf. The 40 m cement plug was drilled out at an average ROP of 10.7 m/h. At 2230 h on 26 April, washing and reaming the hole at ~920 mbsf resumed, and it continued until 0600 h on 28 April with no apparent progress. The driller worked stuck pipe for 1 h during this time interval. The drill string was recovered to change the bit, which had accumulated 33.3 h of work. The used bit (Fig. F19C) was found to be in gauge and exhibited little wear on the cones and bit body. The two junk baskets were emptied (Fig. F19D) and contained basaltic cuttings ranging from fine angular sand to small pebbles. Rare small (as large as centimeter-scale) chunks of cement were also recovered. The quantity of material was noticeably less than that from the previous junk basket deployment (Run 2).

Run 7

A new hard formation Smith Q7JS (IADC code 735) tricone bit (Fig. F19E) was picked up and deployed with the drilling assembly. The nozzles were reduced to inches from the usual inches to increase the downhole hydraulic horsepower. The junk baskets were not added to the BHA to reduce the risk of downhole hardware failure. The bit entered the reentry cone for the seventh time of the expedition at 0135 h on 29 April 2011. Washing and reaming resumed, and perseverance was finally rewarded when the bit passed through the obstruction at 922 mbsf shortly before midnight on 29 April. The pipe got stuck for 2 h at 1162 mbsf. The total depth achieved during Expedition 312 (1507.1 mbsf) was reached at 0915 h on 1 May. A total of 6 m of hard fill was encountered at the bottom of the hole, which was ground and removed before the bore was flushed with a 100 bbl high-viscosity mud sweep. The wiper trip to 890 mbsf was without incident. A pill of 60 bbl of heavy mud was spotted across the zone extending from 920 to 960 mbsf to keep the formation from bridging over the open hole prior to retrieving the pipe trip to the surface.

Run 8

The cementing assembly was deployed for the third time of the expedition with the bit entering the reentry cone for the eighth time at 1300 h on 2 May 2011. The bit was placed below the unstable zone at 960.5 mbsf, and 65 bbl of 15 ppg cement was pumped into the hole. The intent of this operation was to fill and stabilize the washed out and apparently unstable portion of the hole below 922 mbsf (see “Observations of highly out-of-gauge interval (920–960 mbsf)”). This region was not accessible prior to drilling through the ledge. The drill string was flushed with a large volume of seawater prior to withdrawal from the hole. The drill string was tripped to the surface, and the BHA was recovered by 0315 h on 3 May.

Coring (3–7 May)

Run 9

An RCB coring assembly made up with a Rock Bit International (RBI) C-9 hard formation coring bit, 11 controlled length drill collars, a tapered drill collar, six joints of 5½ inch drill pipe, and associated subs was deployed at 0630 h on 3 May 2011. The bit reentered the cone for the ninth reentry of the expedition at 1235 h. Thought was given to penetrating the cement plug with a center bit, but it was decided that coring the cement would be more efficient and keep the annulus cleaner. The driller tagged the cement at 924 mbsf and began coring at 1745 h on 3 May (ghost cores 335-1256D-G1 through G6) (Table T4). At 1330 h on 4 May, the cement plug was penetrated and the remaining portion of the hole was washed (wash core 7W) (Table T4) and reamed to bottom by the early morning of 5 May. Rotary coring in Hole 1256D began at 0145 h on 5 May, which was exactly 16.0 days after arriving on station. Rotary coring advanced from 1507.1 to 1520.2 mbsf (Cores 335-1256D-235R through 238R) (Table T4) by 1200 h on 6 May, using nonmagnetic core barrels without liners to reduce jamming and increase recovery. Coring was difficult with occasional erratically high torque. Overpull of as much as 60,000 lb was frequently needed to keep the drill string free. When the last 2 m of advance required nearly 12 h, the core barrel was recovered and found to contain only three pebbles. Initially, it was thought that the bit throat might be jammed, which is not uncommon in hard rock coring. However, examination of the core catcher sub located at the bottom of the core barrel showed evidence of grinding and abrasion damage, indicating a serious mechanical problem at the bit (Fig. F19F).

The drill string was recovered, with the bit clearing the rotary table at 0545 h. The bit was totally unrecognizable (Fig. F19G, F19H). The body of the bit was honed to a smooth profile at the bottom and on the sides, earning it the name “Stumpy.” The bit was missing all four cones, four legs, and core guides. The bit spiral stabilizer blades and embedded tungsten carbide inserts were also absent. The severity of the damage indicated that the bit had continued to rotate hours after experiencing failure, likely as long as 10 h, based on changes in penetration rates. This spectacular failure was masked by the difficult drilling conditions >5 km beneath the hull.

Cleaning, milling, and reaming (7–26 May)

Run 10

Before coring could resume, the metal hardware debris, “junk,” had to be removed from the bottom of the hole. The first attempt at retrieving the junk was made with a fishing assembly including a Bowen 9 inch fishing magnet (Fig. F19I, F19K) coupled to a tandem set of junk baskets. This assembly was run in with two stands of drill collars, reentering Hole 1256D for the tenth time at 1815 h on 7 May 2011. The fishing assembly was run in without incident to 1295 mbsf, where it contacted a ledge. The top drive was picked up, and the assembly was advanced to within 73 m of the bottom (~1434 mbsf), where circulation was lost. All attempts at unblocking the flow path by varying the pump strokes and running pressures as high as 2500 psi with 20 strokes per minute were unable to clear the blockage. Because it would be reckless to attempt to advance any further down the hole without circulation, the drill string was recovered. The magnet, both junk baskets, and the bit sub were packed with sand-sized basaltic cuttings mixed with metal shavings and some cement cuttings (Fig. F19J). This material apparently worked past the float valve during periods of low fluid flow and circulation breaks while making pipe connections. The consensus was that the lower section of the hole needed to be cleaned out to within a couple of meters of the wreckage before a mill or magnet could be effective.

Run 11

The next fishing attempt was made with a used Atlas tricone bit (IADC type 517), a tandem set of external junk baskets (EXJB), and three stands of drill collars. The fishing assembly reentered Hole 1256D at 0315 h on 9 May 2011. The hole was washed, reamed, and heavily flushed with large-volume, high-viscosity mud sweeps. Special attention was given to reaming and washing the ledges found at 1356 mbsf, from 1459 to 1478 mbsf, and at 1520 mbsf. The bit cleared the seafloor at 0605 h and the rotary table at 1130 h on 10 May. The surface of the reentry cone was flushed prior to withdrawal from the hole. The two EXJBs were found to contain material ranging from fine gravel to cobble-size rocks (Fig. F19L, F19M).

Run 12

The next fishing attempt was made with a 9¾ inch Bowen full-flow reverse circulation junk basket (RCJB) (Fig. F19N, F19O). The principle of reverse circulation increases the chance of recovery of bit cones, tong pins, hammers, and similar debris. The circulating fluid is jetted outward and downward against the full circumference of the hole, where it is deflected and directs loose objects into the long hollow barrel of the basket. The unit is activated by dropping a stainless steel ball from the surface (Fig. F19S) and does not utilize a float shoe. The RCJB with a Type B mill was made up to a single EXJB and a six–drill collar BHA and deployed at 1300 h on 10 May 2011. The assembly reentered Hole 1256D at 2330 h. After the slow circulating parameters were obtained and the hole was flushed with a 100 bbl high-viscosity sweep, the stainless steel activation ball was dropped into the open pipe and the tool lowered to within 1 m of the bottom (~1516 mbsf). Because of the high pump rates and resulting standpipe pressure employed, the landing of the ball in the RCJB was not immediately obvious. The tool was worked for 30 min before the drill string was recovered. On the surface, all five 8¼ inch drill collars were found to be filled with fine hole cuttings weighing on the order of a few hundred kilograms (Fig. F19P). Coarser gravel was also found in the lower part of the BHA in the head sub, crossover subs, and bit subs. The RCJB basket (Fig. F19Q, F19R) contained an array of granoblastic dike rocks with 20 weighing >100 g. One specimen was 20 cm × 10 cm × 10 cm and weighed 4.5 kg. The total weight of all rocks in the basket was estimated at ~20 kg. The large rocks were assumed to be covering whatever remains of the bit cones, bit legs, and core guides at the bottom of the hole.

Run 13

The second deployment of the Bowen RCJB was made with a Type A mill shoe and reentered Hole 1256D at 1335 h on 12 May 2011. The tool was run in the hole to 1385 mbsf, where the top drive was picked up. The assembly was carefully advanced with rotation and circulation and tagged a hard ledge at 1518 mbsf but was unable to advance past this depth. The steel ball was dropped, but activation of reverse circulation was obscured by pump pressure as high as 3000 psi at 50 spm. The drill string was recovered, with the RCJB clearing the top of the cone at 0340 h on 13 May. Once on the surface, the BHA was again found filled with fine cuttings with a volume comparable to the previous run (Fig. F19T). The RCJB basket contained two rocks with a total weight of 4.5 kg. The RCJB was cleaned, dressed, and laid down.

Run 14

The forth fishing attempt was made with a 9½ inch Homco flow-through junk basket (FTJB) (Fig. F19U). The FTJB does not employ reverse circulation and has a deeper throat than the RCJB. The FTJB is also deployed with a float shoe that reduces the potential for inadvertent filling of the inside of the BHA with cuttings. The fishing assembly entered the reentry cone at 2315 h on 13 May 2011 and was run in to 1521 mbsf by 0815 h. The driller slowly worked the tool on bottom for 30 min before pulling back. Mud flushes totaling 300 bbl were circulated during this process. The drill string cleared the seafloor at 1345 h and was recovered by 2015 h on 14 May. The FTJB contained two rocks of granoblastic dike origin that weighed a combined 3.2 kg. Of the two sets of junk catcher fingers, the lower set was completely devoid of fingers (Fig. F19V). Although the FTJB completed the trip to the bottom of the hole, only a fraction of the material snared by the tool was apparently captured.

Run 15

After reviewing the available options, it was decided that the best way to proceed was to reenter the hole with a hard-formation tricone drilling assembly and attempt to grind away the ~2 m of hard fill presumably overlying the metal debris of the failed core bit. Once the hard fill was removed, the metal debris could be milled down or possibly recovered by the fishing magnet. Given the persistence of cobble-sized material near the bottom of the hole, indicated by the contents of the RCJB of Run 14, a new Smith 7JS tricone bit was picked up and fitted with 3X15 nozzles and affixed to a three-stand BHA with the goal of grinding up the remaining loose rock. The drilling assembly reentered Hole 1256D at 0730 h on 15 May 2011. Hard contact was made at 1518.8 mbsf. From 1415 h on 15 May until 0615 on 16 May the hole was washed and reamed from 1518.5 to 1521.1 mbsf and flushed with 460 bbl of high-viscosity mud sweeps. (Note: the precision of these depths is ±1 m, as high tides at Site 1256 precluded accurate determination of the bit depth.) The bit was tripped to the surface after 14.5 rotating hours to ascertain the condition of the bit and assess the progress. The bit cleared the seafloor at 1015 h and the rotary table at 1545 h on 16 May. The cones exhibited virtually no wear except for a chipped insert on the gauge cutter. The bearings were tight with some apparent shirttail wear and minor junk damage present on the bit body (Fig. F19Y). These were characteristics of a bit that had done very little actual drilling. When the bit diameter was measured, it was found to be under gauge by inches (Fig. F19W). Apparently the bit had been literally squeezed into a smaller diameter hole (<10 inches in diameter). The conclusion was that the bottom ~3 m of the hole was considerably undersized and would have to be reamed to full gauge before fishing could resume. One of the EXJBs was significantly damaged (Fig. F19X).

Run 16

The sixth tricone bit used during the expedition was selected based less on cutting structure and more on the amount of armor on the legs, because that would be the area that would receive most of the wear during reaming. A Smith FH3VPS 9⅞ inch tricone was made up to a three-stand BHA without external junk baskets. The bit reentered Hole 1256D for the sixteenth time of the expedition at 0245 h on 17 May 2011. By 0815 h, washing and reaming operations began. The undergauge section of the hole ranging from 1516.5 to 1519.7 mbsf was reamed for 15 rotating hours and flushed with a total of 260 bbl of high-viscosity sepiolite sweeps. The bit was pulled clear of the seafloor at 0340 h on 18 May and recovered at 0900 h. The tricone bit was found to be in gauge. Although the tricone was missing six teeth on the middle row of one cone, it was in reasonably good condition (Fig. F19Z, F19AA). The missing teeth suggested that cone contact might have been made with debris at the bottom of the hole. During the reaming, care was taken to avoid penetrating below 1520 mbsf to prevent metal to metal contact with the RCB bit wreckage and avoid potentially leaving more junk in the hole.

Run 17

Once the reaming was concluded, a flat-bottomed 9⅝ inch mill was made up to the three-stand BHA with a bit sub junk basket (BSJB) and deployed. The mill entered Hole 1256D at 1850 h on 18 May 2011 and initiated milling at 0130 h on 19 May. Shortly after reentering the cone for the seventeenth time during the expedition, the total length of drill pipe tripped during the cruise passed 100 miles. This milestone was acknowledged by a message of thanks to the Transocean teams from the Shipboard Science Party. Milling at a depth of ~1520–1521 mbsf progressed without incident until 1330 h. The driller frequently picked up the tool a little off bottom and decreased the pump pressure to let cuttings settle and be captured in the EXJB. This is referred to as “working the junk baskets.” A total of 300 bbl of mud was circulated to flush the hole during milling. The mill cleared the seafloor at 1920 h on 19 May and was recovered by 0315 h the next morning. The trip out of the hole was suspended for 1.5 h for the sixth slipping and cutting of the drilling line. The abrasive hard surface on the face of the mill was completely worn away, and the diameter of the tool was under gauge by 0.5 inches (Fig. F19BB). The BSJB was also found damaged (Fig. F19CC).

Run 18

The second milling tool run was made with a 9 inch flat-bottom mill (Fig. F19DD), which reentered the hole at 1415 h on 20 May 2011 for the eighteenth time. From 1945 h until 0145 h on 21 May the bottom of the hole was milled and the junk basket worked. Based on the excessive wear to the first milling tool, the rotating time for the second mill run was scaled down from 12 h to 6 h. A total of 320 bbl of sepiolite mud was circulated to keep the hole clean. The drill string was pulled free of the seafloor at 0645 h, and the second milling tool was secured on deck at 1225 h on 21 May. The milling surface was abraded clean, and some minor junk damage was noted on the side of the mill (Fig. F19FF). There was also a 3 inch, 200 circumferential grove cut into the crossover sub located just above the mill. The BSJB was unloaded and found damaged (Fig. F19EE). It contained the usual small cuttings and gravel with some small fresh cuttings of metal. This was the first suggestion that we had finally started to mill bit debris.

Run 19

The next fishing attempt was made with the third deployment of the RCJB (Fig. F19GG), together with two EXJBs and one BSJB (Fig. F19HH). This assembly reentered Hole 1256D at 2230 h, on Run 19 of the expedition. The tool was run to ~3 m off bottom and the junk baskets worked for ~10 min with pump strokes as high as 150 spm and standpipe pressure reaching 1850 psi. Following a 100 bbl mud flush, the RCJB was worked to the bottom (~1521 mbsf) with minimum rotation and very light weight on bit. The string was pulled out of the hole, clearing the seafloor at 1015 h and the rotary table at 1645 h on 22 May 2011. The RCJB was filled with congealed sepiolite in which were four large rocks with a total weight of 8.9 kg (Fig. F19II). The largest sample weighed 3.9 kg. The external junk baskets contained fine cuttings, small pebbles, and a few tiny metal fragments.

Run 20

The RCJB was rebuilt and deployed for the fourth time of the expedition along with the three junk baskets and reentered Hole 1256D at 0635 h on 23 May 2011 for the twentieth time on this expedition. The trip in was extended for 3 h while the drill crew repaired the pneumatic control lines for the high clutch in the drawworks. Following the routine of working the junk baskets and flushing the hole with 100 bbl of sepiolite mud, the RCJB was activated and advanced to the bottom of the hole at ~1521 mbsf. The RCJB assembly was pulled free of the seafloor at 1725 h on 23 May and recovered on deck by 0215 h on 24 May. The RCJB (Fig. F19JJ) contained three rocks with a total weight of 5.0 kg. The angularity of the rocks indicated that they were freshly deposited with a suspected origin somewhere in the bottommost 7 m of the hole. One rock (1.4 kg) was a gabbro, suggesting that the “promised land” might be tantalizingly close. The junk basket contained the usual suspects, ranging from gravel-sized cuttings to small pebbles with a few metal filings distributed throughout.

Run 21

The fifth and last RCJB run of the expedition was deployed with the three junk baskets at 0545 h on 24 May 2011. The tool entered the reentry cone at 1115 h for the twenty-first reentry of the expedition. After working the junk baskets and circulating 100 bbl of sepiolite mud, the RCJB was activated and succeeded in reaching the bottom of the hole at ~1521 mbsf. The RCJB was retracted at 1845 h and the bottom of the hole displaced with 200 bbl of drill water for logging purposes. The RCJB cleared the seafloor at 0100 h following a 1 h interruption to replace a damaged cam roller on the dual elevator handling system. The drill string was recovered at 0700 h on 25 May. The junk baskets contained the usual collection of fine cuttings and small gravel. The RCJB contained four small rocks. The trend of the progressively smaller-sized rocks and lighter yield for each succeeding run of the RCJB indicated that the rubble pile at the bottom of the hole had been successfully removed. However, the lack of significant metal in all junk basket runs was perplexing. It was decided to delay the start of the logging program and attempt to recover any metal debris at the bottom using the fishing magnet.

Run 22

The 9 inch Bowen fishing magnet was made up along with two EXJBs and one BSJB and deployed. The fishing assembly entered Hole 1256D at 1645 h on 25 May 2011. After working the junk baskets and fishing magnet at the bottom of the hole for 30 min, another 200 bbl of drill water was pumped into the bottom of the hole. The routine 100 bbl mud sweep was omitted to save time. The fishing magnet cleared the seafloor at 0230 h on 26 May and was on deck at 0900 h. The junk baskets contained the routine assorted samples of cuttings and pebbles. The magnet contained only small filings and minor amount of metal (Fig. F19KK).

Logging (26–28 May)

Run 23

A lightweight logging assembly was made up comprising a 9½ inch logging bit, a landing saver sub, a controlled length drill collar, a tapered drill collar, five transition joints of 5½ inch drill pipe, and a crossover sub. The logging assembly entered the cone at 1725 h on 26 May 2011 for the twenty-third reentry of the expedition. The bit was placed at a depth of 218.9 mbsf.

The first log was the triple combo (natural gamma ray logging tool [GR]/Accelerator Porosity Sonde [APS]/Hostile Environment Litho-Density Sonde [HLDS]/High-Resolution Laterolog Array [HRLA]/GPIT), which was deployed into the pipe at 2255 h and recovered at 0700 h on 27 May. The tool successfully reached the bottom of the hole, although the caliper did not open until ~1490 mbsf. The bottom of the hole, not logged during Expedition 312, was logged using standard procedures up to ~1300 mbsf. A caliper log for the rest of the hole right up into the rat hole was measured to assist planning of cementing operations at the end of the cruise. The triple combo tool returned with three damaged bowsprings on the upper centralizer (Fig. F19LL), and these were replaced in preparation for the second scheduled logging run with the FMS-sonic tool.

The FMS-sonic tool string was initially deployed into the pipe at 1050 h but had to be retrieved at 1410 h when it was unable to exit the BHA into the borehole because of some mechanical obstruction at the bit. Once on the surface, one damaged bowspring of the lower centralizer was replaced in preparation for the second run. The FMS-sonic was redeployed at 1500 h and again experienced difficulty exiting the BHA. On this occasion the tool became irretrievably stuck while partially outside the pipe (Fig. F19MM). The Kinley crimping and cutting tool had to be utilized to sever the logging cable. The loose end of the logging cable was recovered at 0330 h and secured. The BHA with ~20 m of logging tool extending below the logging bit was carefully withdrawn from the reentry cone at 0425 h.

Coring and Cementing (28–30 May)

Run 24

After the BHA was recovered and the FMS-sonic centralizer extracted from the landing saver sub (Fig. F19MM), a three-stand RCB coring assembly was made up with a new Ulterra 9⅞ inch RCB bit (Fig. F19NN). The last activity of the expedition was to run in with the RCB assembly and deposit cement plugs at the bottom of the hole and from 910 to 940 mbsf. Before pumping the cement, it was decided to core the bottom of the hole until the core bit debris left on 6 May stopped progress or the little time remaining expired, whichever came first.

The RCB assembly entered the reentry cone for the twenty-fourth time of the expedition at 2225 h on 28 May 2011. A fresh core barrel was deployed at 0515 h on 29 May, and by 0545 h coring resumed in Hole 1256D. The core barrel was recovered after advancing from 1520.2 to 1521.6 mbsf. The nominal recovery for the 1.4 m advance was 0.5 m (36%) and consisted of 10 large pebbles (Core 335-1256D-239R) (Table T4). The average ROP was 0.6 m/h. The total average recovery for the 14.5 m of coring during this expedition was 11%. There was no metal in the core barrel or signs of junk in the coring process. This, together with the absence of significant metal debris recovered by the fishing magnet during Run 22, was taken as an indication that metal debris had possibly been mostly ground during Run 9, shortly after the failure of the C9 coring bit, and that the hole is now likely clean of significant metal junk.

Time for coring had expired, and cementing operations began at noon. The first cement plug was emplaced at 1521 mbsf (15 bbl), and the second plug was placed from ~940 to 910 mbsf (60 bbl). These plugs will help stabilize the two problem regions in the hole and facilitate return to the bottom of the hole and coring on a future return to Hole 1256D. The C-9 bit used for the last run of Expedition 335 returned to the rig floor in relatively good condition and nearly in gauge (Fig. F19OO). A few teeth were missing (Fig. F19PP), the shirttails were worn, with abrasion marks (Fig. F19QQ), much smaller than those caused by metal junk observed during Expedition 312 (Fig. F19RR).

Observations of highly out-of-gauge interval (920–960 mbsf)

This section summarizes geological and wireline knowledge regarding the problematic region between 920 and 960 mbsf in Hole 1256D that impeded the progress of the drill string to the full depth of the hole on both Expeditions 312 and 335. It seems prudent to collate geological and engineering information on this trouble spot to inform future drilling operations in Hole 1256D. Figure F20 summarizes the stratigraphy of Hole 1256D following operations during Leg 206 and Expedition 309/312. Attention is drawn to the caliper log for the hole. Throughout the lava sequences, the hole diameter is strongly out of gauge between 25 and 50 cm (10 and 20 inches) diameter, reflecting the fractured nature of the formation and continued erosion resulting from numerous pipe trips through the lava horizons. Although there is an out-of-gauge interval at the bottom of the transition zone (from 1050 to 1060 mbsf), as of the end of Expedition 312 the hole through the sheeted dike and uppermost plutonic complexes is close to expected gauge. The changing caliper of the borehole is well illustrated in Figure F21, which shows the deviation of Hole 1256D (~60 m due west at 1400 mbsf). The large caliper of the hole at the bottom of the lava sequences is clearly visible. Figure F22 presents the series of caliper logs from 900 to 1000 mbsf successively recorded during Expeditions 309/312 and 335.

Hole blockages were encountered in Hole 1256D in the interval from ~920 to 960 mbsf during the initial reentries during both Expeditions 312 and 335, which prevented the transit of the drill string to the bottom of the hole. Difficult hole conditions were not expected during Expedition 312, which occurred only 2 months after the successful deepening of Hole 1256D to 1257 mbsf during Expedition 309. On reentering Hole 1256D, the drill string advanced without incident until resistance at 927 mbsf prevented further progress. Approximately 5 days of Expedition 312 were spent reaming and cleaning the interval from 927 to 1051 mbsf, with the region of the hole from 927 to 944 mbsf being very tight and receiving the most attention.

The initial reentry into Hole 1256D during Expedition 335 was impeded by an obstruction at ~922 mbsf. A total of 15 days were then spent clearing this obstruction, stabilizing the zone from 910 to 960 mbsf, and cleaning and reaming the hole to its full depth (1507 mbsf at the end of Expedition 312).

Expedition 309 coring operations for the interval from 900 to 980 mbsf

Coring operations between ~897 and 980 mbsf (Cores 309-1256D-97R through 111R) are briefly reviewed herein. The interval from 897.8 to 958.8 mbsf (4543.2 to 4604.2 mbrf) was drilled without incident during Expedition 309, with moderate recovery (24.3%) and a ROP of 1.15 m/h (RCB Bit 3) (Fig. F23). Before coring commenced with the next bit, a bottom seawater sample was taken with the WSTP. The hole was then reentered and reamed from 868.6 mbsf (4514 mbrf) to the bottom, clearing ~4 m of fill at the bottom of the hole. Coring continued without incident to 974.4 mbsf with a ROP of 1.14 m/h and good recovery (46.7%) (4619.8 mbrf), until a drop in standpipe pressure following the retrieval of Cores 309-1256D-110R and 111R alerted the drillers to major damage of the bit sub (see the Operations section in Expedition 309/312 Scientists, 2006). The only operational irregularities apparent in the coring of this interval during Expedition 309 was that two cores (309-1256D-101R and 102R) were cored as 9 m advances as opposed to ~4.8 m half-cores that had been the standard advance since Core 309-1256D-90R. However, these longer advances were not drilled at high rates of recovery as might have occurred if these increased advances had encountered softer formations (e.g., hyaloclastite, flow-top breccia).

The rocks recovered in this interval are predominantly aphyric microcrystalline to fine-grained basalt sheet and massive flows (Fig. F23). Rare volcanic glass, chilled margins, breccia, and altered hyaloclastite were recovered. The lavas are only slightly altered, but there are very common 0.1 to 1 mm saponite ± pyrite veins with minor silica and chalcopyrite and rare celadonite, iron oxyhydroxides, and carbonate. Saponite veins are commonly flanked by <5 mm black halos and rarely by green-gray or brown halos. These observations are consistent with a fractured and potential fragile formation. However, the rocks recovered are not especially different from any other parts of the lava sequences. A possible interpretation of the problems encountered in this area at the beginning of Expedition 335, tentatively sketched in Figure F24, is that a slab of a massive lava flow surrounded by more fractured intervals detached and partially obstructed the hole, wedging the BHA against the wall. This situation would explain the strong overpull required to free the pipe when it got stuck in this interval (Table T3).

Wireline geophysical logs clearly identify this interval between 920 and 960 mbsf, which exhibits out-of-gauge caliper and low resistivity. Temperature measured by wireline following the conclusion of drilling operations during Expeditions 309/312 and 335 display clear negative excursions (Figs. F11, F23). This suggests that, within this zone, the incursion of cold drilling fluid occurs during coring and cleaning operations, and this incursion inhibits the recovery of this section back to thermal equilibrium following the conclusion of drill fluid pumping.